1. Field of the Invention
The invention relates to a pile hammer comprising a cylinder, a piston displaceably guided in the cylinder and a striker displaceably guided in the cylinder. The invention furthermore relates to a method for operation of a pile hammer.
2. Description of the Related Art
Pile hammers, which are regularly operated with diesel oil in the state of the art, for which reason they are also called diesel hammer pile drivers, diesel hammers or diesel pile drivers, are particularly used in foundation work in the construction industry. The pile hammers are used for driving posts of all kinds, such as concrete pillars, iron beams, sheet pile wall elements or the like into a construction ground.
To start such a diesel hammer pile driver, the piston is pulled upward within the cylinder, using a disengagement apparatus, and disengaged at a specific height, thereby dropping downward onto the striker, under the effect of gravity. As it drops, the piston activates a fuel pump, by way of which feed of fuel, particularly diesel oil, takes place by way of one or more injection nozzles. The air situated in the combustion chamber of the cylinder is compressed by the dropping piston, and thereby heated so that the fuel/air mixture present in the combustion chamber is ignited, whereupon it combusts in the manner of an explosion. As a result of the explosion energy released during this process, for one thing the piston is accelerated back upward for a new work cycle; at the same time, the material being pile-driven is driven into the ground by way of the striker.
In the case of such diesel hammers, two types are known, with different methods for injecting diesel oil into the combustion chamber. In the case of what is called high-pressure injection, the fuel is injected into the combustion chamber of the cylinder, at high pressure, in the form of a finely atomized fuel mist, during compression of the air by the dropping piston. This mist, together with the air, forms an ignitable mixture. In the case of high-pressure injection, the fuel already ignites during the compression process, as soon as the compressed air reaches a temperature that suffices to ignite the fuel mixture. As the result of the explosion-like combustion, a high pressure is built up in the combustion chamber, by means of which the piston is braked, on the one hand. On the other hand, this combustion pressure acts on the striker, which exerts a force on the material to be pile-driven, thereby driving material this into the ground.
The compression process ends, at the latest, with the impact of the piston on the striker, whereby the piston, which after all was already braked before impact on the striker, by the expanding combustion products, does not impact the striker with full kinetic energy. At times, particularly in the case of a hard construction soil, the case can actually occur that the piston does not touch the striker at all and is accelerated upward again by the combustion gases, without any prior contact with the striker. Under such conditions, the striker acts on the material to be pile-driven only by way of the combustion gas cushion. For this reason, diesel hammers in which high-pressure injection is used are less suitable for driving heavy material to be pile-driven or in the case of difficult soil conditions with hard layers.
Furthermore, such a diesel hammer becomes very hot during operation, and the system of high-pressure injection tends to misfire when overheated. Such a system is furthermore susceptible to failure and has a relatively complicated structure. As a result, a diesel hammer with high-pressure injection has the disadvantage that it cannot be easily repaired or cannot be repaired at all on site, at construction sites.
Advantages of high-pressure injection can be seen as its good, relatively residue-free combustion, good starting behavior of the diesel hammer, as well as a good pile-driving effect in the case of soft soil layers.
In the case of what is called impact atomization, which is also called low-pressure injection, in contrast to high-pressure injection, the fuel is introduced into the combustion chamber at the beginning of the compression process, at a lower pressure, in the form of a fuel jet, and afterwards lies on the upper face side of the striker as a fuel puddle at first. The air in the combustion chamber is compressed by the dropping piston until the piston impacts the striker. At this moment, the liquid fuel is atomized by the impacting piston surface and ignites in this state, in the hot, compressed air. The piston is then accelerated upward by the explosion, whereupon a new work cycle can begin.
Until the impact onto the striker occurs, the piston is braked in its drop merely by the air situated in the combustion chamber and compressed by the piston. As a result, the movement energy of the piston is transferred to the striker, for the most part, thereby making it possible to exert clearly greater impact forces to the material to be pile-driven, at the same weight of the piston, than is the case for high-pressure injection. The impact of the piston on the striker takes place before the combustion of the fuel, in terms of time.
Diesel hammers that use low-pressure injection are less well suited for being used at low soil resistance values. In this case, the compression based on the low resistance of the soil is reduced, because the compression pressure that builds up is already transferred to the material to be pile-driven by way of the striker moving downward. The combustion chamber is thereby actually enlarged, and this enlargement in turn is at the expense of the compression pressure. In the case of soft soil, combustion takes place only at reduced quality, which can lead to undesirable residues (soot, non-combusted fuel in the combustion gases), which are a burden on the environment.
An advantage of impact atomization is that the movement energy of the piston is effectively utilized, because the piston has a hard impact on the striker. Furthermore, a diesel hammer using impact atomization has a lesser tendency to overheat, is less susceptible to failure, and is easier to operate than a diesel hammer using high-pressure injection.
The disadvantage of diesel hammers was that a diesel hammer operating according to one of the two working principles could only take specific local conditions into account. Up to the present, this disadvantage had to be accepted. If it turned out, on site, that the soil composition was or became different from what was planned in advance, either the work had to be continued with the non-optimal device or a different diesel hammer had to be procured, leading to loss of time and higher costs.
In WO 2006/072297 A1, a diesel hammer is described, in which the diesel oil can be injected into the combustion chamber optionally as an atomized fuel mist (high-pressure injection) and/or as a fuel jet (low-pressure injection). This diesel hammer has proven itself in practice. In the case of this diesel hammer, it is possible to operate it with high-pressure injection in the case of soft soil conditions, but with low-pressure injection or impact atomization in the case of hard soil layers. Thus adaptation of the effectiveness of the diesel hammer, with simultaneous optimization of combustion, to soft or hard layers of the soil is guaranteed.
In practice, however, it has been shown that incomplete fuel combustion can occur in the different operating modes, thereby causing combustion residues to remain in the combustion chamber.